Share Tea With Saturn In Sagittarius

Sagittarius represent a centaur — half man and half horse. The constellation is very ancient, tracing its origin to Sumerian times, when it represented a god of war and hunting called PA.BIL.SAG. But instead of a cloak, he had wings. Sidney Hall / Urania’s Mirror

The same way we used Jupiter to find the constellation Scorpius a few weeks back, we can use Saturn to find Sagittarius the Archer. Better known as the Teapot from its shape, the constellation never gets very high in the sky viewed from the northern United States and Canada. For me, the bottom half of the Teapot doesn’t rise above the trees until it’s due south and highest in the sky. But after Orion, Ursa Major and a few others, Sagittarius remains one of my favorite constellations not only for its shape but also because there are a lot of wonderful things to look at here in binoculars or a telescope.

Find Sagittarius by locating Jupiter, a bright “star” in the southern sky. Then look 3 fist to the left and a little below Jupiter to spy Saturn. The Milk Dipper asterism glimmers 5° below Saturn and forms the handle of the Teapot. Stellarium

Sagittarius is also home to a bright section of the Milky Way and the center of the galaxy itself. To find it, face south around 10:30 p.m. in late July. The first thing you’ll see is the brilliant planet Jupiter about a third of the way up in the sky. Now, reach out a balled fist and put Jupiter at one end and look three fists to the left and below. You should see another bright “star” — that’s Saturn.

Once you’ve got Saturn in view, reach your hand out at arm’s length, but this time look three fingers (held together) below the planet and you should see a small trapezoid of stars. If you include the star just above and to the right of the figure, you’ll see it forms a little dipper. In fact, it’s an asterism or star-pattern nicknamed the Milk Dipper. That name comes from the shape and its location not far from the band of the Milky Way.

Sagittarius lies in a rich region of the sky crossed by the band of the Milky Way. It’s a fantastic place to explore with binoculars on a moonless night. The dark patches that cut into the band are great clouds of cosmic dust that block the light of more distant stars. Many of them dapple the Milky Way above the spout. Bob King

Pictured another way, it forms the handle and top of the Teapot. With the Milk Dipper under your belt, direct your gaze below and right of Teapot top to find the triangular “spout,” then connect spout and dipper to complete the pot’s outline. Sagittarius’ second brightest star Nunki (NOON-kee) shines from the top of the handle. The name is an ancient one, harkening back to the Babylonians to whom NUN-KI referred to a group of stars representing their sacred city of Eridu on the Euphrates River. Later, the name was applied exclusively to this particular star in the constellation.

Notice that the Milky Way flows over the top of the Teapot and past its spout. Because the band is low in the sky from mid-northern latitudes skywatchers in the northern half of the U.S. and much of Europe its light — and gloriousness — is often absorbed by haze. Here again is another excuse for traveling to the Caribbean or the southern hemisphere. The better to see the Sagittarius Milky Way!

The Milky Way is a spiral galaxy with a prominent dense bar of stars across its core. The sun (yellow dot) and planets are located with a spiral arm some 26,000 light years from the center. During evening hours in northern hemisphere summer, we face toward the galaxy’s center. In winter, we look outward across the outer spiral arms into intergalactic space. NASA/JPL-Caltech/R. Hurt (SSC/Caltech)

When you face the Teapot you’re looking toward the center of the Milky Way galaxy 26,000 light years away. Billions of stars and a host of cosmic dust clouds lie between us and the core. The stars pile up to form the thick ribbon of hazy light — the band of the Milky Way — while the dust blocks the light of distant stars, creating dappled patches of darkness (see photo above) within the band. We see a dense band of stars instead of random stars all over the sky because our solar system is located within the Frisbee-like disk of the galaxy. When we look through the Frisbee — along its plane where the stars are concentrated — the stars pile into a hazy braid. When we look outside that band, the stars thin out rapidly.

The supermassive black hole at the center of our galaxy is named Sagittarius A* (Sagittarius A star) and estimated to contain about 4 million times the mass of our sun. This combination X-ray and infrared photo shows glowing gases and stars circling the (invisible) black hole. X-ray: NASA/UMass/D.Wang et al.; IR: NASA/STScI

The very center of the galaxy, shown by the red dot in the diagram above, sits just off the spout. It would shine with great brilliance were it not for intervening dust. Astronomers using radio and infrared telescopes have literally peeled back the dusty curtain and discovered what lies at the core — a supermassive black hole with a mass equal to 4 million of our suns and a diameter of 14.6 million miles. Many larger galaxies like our own have massive black holes in their cores.

In a future installment we’ll explore some of Sagittarius’s many bright nebulae and clusters visible in nothing more than a pair of binoculars. For now, head out the next clear night and get familiar with the constellation. Remember it’s low in the sky for many of us, so try to find a place with a good view to the south.

6 Responses

  1. Chuck Arand

    From the diagram I understand the different parts of the Milky Way that we see in different seasons, but when do we look up or down out of the Milky Way? For example, does the Andromeda galaxy lie above or below the plane of the Milky Way galaxy? Or put another way, when I see the Andromeda Galaxy in the sky, in which direction am I looking out of the Milky Way? Thank you, Chuck

    1. astrobob

      Hi Chuck,
      It’s a little tricky to know whether you’re looking above (say north) or below (south) because the plane of the solar system does not lie in the plane of the galaxy. Instead it’s tilted 60° with respect to that plane. As a result, the north galactic pole is in Coma Berenices near the bright star Arcturus. The south galactic pole is in the constellation Sculptor which is low in the south on autumn nights as seen from the northern hemisphere. Anytime you look outside the band of the Milky Way — whether above or below — you’re looking above or below the plane of the galaxy. Again, north and south are tricky. The only way to know for certain is to find the objects “galactic coordinates.” The Andromeda galaxy’s galactic LATITUDE is -21.5°, so it’s below the plane of the galaxy. If you download a star plotting program like Stellarium (free at stellarium.org) it will give the galactic latitude and longitude of every object you click. In time, you will be able visualize the cockeyed angle of our solar system with regard to the galactic plane. I hope this helps.

    1. astrobob

      You’re very welcome, Chuck. Your question has prompted me to start a blog about this that will appear sometime in the next week or two.

  2. Chuck Arand

    That sounds great! I look forward to reading it.

    If I my ask another question, how can I develop the skill of estimating the apparent brightness of stars (I appreciated your magnitude of brightness scale on page 62) when looking at a constellation without having to resort constantly to a star atlas (assuming that it is possible to develop such skill)? Chuck

    1. astrobob

      Chuck,
      That is a really great question. All it takes is familiarity with with certain reference stars. For the summer sky you might try Vega (magnitude 0), Deneb (mag. 1.2 ) or Antares (1.0), the three stars that form the western half of the W of Cassiopeia (all about mag. 2) or Polaris (mag. 2.0), Gammi Sagittarii (the star at the end of the Teapot’s spout (mag. 3), Delta and Zeta Lyrae (the two stars just left and below Vega, both about mag. 4) and Theta in the Little Dipper (mag. 5.0). To go fainter or to find fractional magnitudes like 4.5 and 1.6, I think the best thing would be to download Stellarium (Mac or Windows, free) at stellarium.org. Click on any star and it will show its magnitude in the info box. Then you can just roam the sky and select stars of certain magnitudes you want to see, in the process becoming more familiar with star brightnesses. After a little time you’ll be able to estimate magnitudes easily.

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